The invention further relates to a system coprising a hydraulic actuator and a device for controlling the actuator.
U.S. Pat. No. 4,649,704 relates to a subsea valve actuator device which is connected to a pressurized fluid accumulator. The pressurized fluid accumulator consists of two hydraulic cylinders with movable pistons which have the same diameter, and which are interconnected via a piston rod.
U.S. Pat. No. 4,240,463 relates to a hydraulic actuator with a fluid accumulator for opening or closing a safety valve, comprising a cylindrical chamber with a movable piston.
U.S. Pat. No. 4,087,073 relates to a safety valve which is combined with a hydraulic actuator. The actuator comprises a cylindrical chamber with a movable piston similar to that which is described in U.S. Pat. No. 4,240,463. The piston position and movement are dependent on a fluid pressure which is controlled by a pilot valve.
EP A2 38 034 relates to a safety valve-manifold system for opening or closing a subsea safety valve. This system comprises a hydraulic control line which is connected to a pressurized fluid accumulator.
Such known systems, therefore, may comprise hydraulic devices such as hydraulic actuators, which can be supplied with a pressurized fluid, and from which a return fluid may flow, these fluids flowing in separate lines. The device may be a hydraulic cylinder with a cylinder part, wherein a piston part can be moved. The piston part together with the cylinder part may define two chambers on the respective sides of the piston part, where the pressurized fluid can influence one chamber and the return fluid can influence the other chamber. An example of such a device is a balanced, hydraulic valve actuator, whereby the position for a valve, especially a process valve, can be set. A hydraulic actuator should be understood to also cover hydraulic motors.
A typical drawback of such hydraulic systems is that the fluid supply and return lines influence the dynamic characteristics of the system in such a manner that the system""s time constants and thereby the response or operating time for the hydraulic actuator is increased when the lines for supply and return of fluid respectively are long, which is the case, for example, when controlling a hydraulic actuator for a subsea valve of an oil or gas well.
In order to control an oil or gas flow from a well, the well is provided with a wellhead Christmas tree comprising process control valves, where each process control valve is provided with an actuator for operation of the valve. The actuator may be operated electrically or hydraulically, but hydraulically operated actuators are normally employed for wellhead Christmas trees in oil and gas production.
In the case of so-called open systems, hydraulic actuators such as a hydraulic cylinder may be employed to which there extends only one single line, which is connected to one of the chambers. The second chamber communicates with the space surrounding the actuator, and in this chamber there may be mounted a return spring which attempts to move the piston part towards the first chamber. In order to move the actuator""s movable part towards the second chamber, the line is supplied with a pressurized fluid whose pressure is so great that a force is generated which is greater than the force which is exerted by the spring. In order to move the movable part in the other direction, the pressure on the fluid in the line is reduced, whereby the return spring causes movement of the movable part in the other direction while at the same time the fluid is forced back in the line. In open systems, hydraulic fluid can easily leak into the surroundings, an occurrence which should be avoided both out of consideration for the environment and on account of the cost involved in the loss of fluid.
Closed hydraulic-valve actuator systems have therefore been developed for oil and gas production of a balanced type, where the actuator is equipped with a cylinder and a piston which define a first and a second cylinder chamber, where a supply line is connected to the first chamber and where a return line is connected to the second chamber. The cylinder may also be equipped with a return spring which moves the piston to a position, which usually corresponds to the process valve""s closed position when the pressure is reduced in the supply line. When pressurized fluid is supplied to the actuator via the supply line, return fluid is simultaneously forced back in the return line. This eliminates the difficulties with regard to fluid discharge compared with the above-mentioned, open system.
The supply and return lines are normally installed in a so-called umbilical, usually together with other hydraulic, electrical and/or optical lines, and/or lines for the supply of other fluids. The umbilical extends from the subsea process control valve near the well to, e.g., a platform. The umbilical may be very long, in some cases up to 20 km or more. In the umbilical it is desirable to employ hydraulic lines with a limited cross section. For construction, installation and cost-related reasons, furthermore, it is an advantage to employ the same cross section for several hydraulic lines in the umbilical. A typical diameter for the umbilical may be approximately 5 cm, and the hydraulic lines which are installed in the umbilical may have a diameter of approximately 12 mm. The flow resistance in the hydraulic lines then becomes substantial, with the result that the response time for the valve actuator in the system may be unsatisfactorily long.
On the basis of the above, it is desirable to provide a device in a subsea system for controlling a hydraulic actuator, and a subsea system for controlling a hydraulic actuator, where the response time for the actuator is satisfactory.
Attempts have been made to achieve this by various means.
A first, known method is to employ hydraulic lines with a larger cross section. The umbilical""s cross section thereby also becomes large, resulting in a substantial increase in costs.
A second method is to connect an accumulator to the supply line near the process control valve. When a valve actuator has to be activated, electrical energy is used to open a solenoid valve mounted between the accumulator and the valve actuator. The accumulator may be a replaceable, precharged tank. However, the actuator is more commonly refilled with fluid from the platform, preferably through a supply line in the umbilical.
By this means the actuator""s response time is reduced, since the difficulties with regard to the supply of a fluid flow through the supply line are partially overcome. However, the return fluid still has to be caused to flow through the return line, with the result that a satisfactory result is not obtained if the supply and return lines have the same cross sectional dimensions. Moreover, the refilling of the accumulator presents difficulties, and when the pressure in the accumulator is too low, the actuator cannot be activated for opening the valve. Furthermore, the installation of high-pressure accumulators on the seabed can be associated with major constructional and installation-related difficulties.
A third, known method is to mount a return pressurized fluid accumulator near the actuator, in connection with the return line. As is known, pressurized fluid accumulators may comprise a housing, wherein there is mounted a movable body, such as a membrane or a piston, one side of which is influenced by a resilient or elastic device, such as a compression spring or a pressurized gasxe2x80x94usually nitrogen gas, and the other side of which in this case may be influenced by the fluid in the return line. Fluid can thereby flow rapidly from the actuator""s return chamber to the return accumulator, with the result that the actuator""s response time is satisfactory. The return fluid is then caused to flow on from the return accumulator up through the return line in the umbilical by an expansion of the compression spring or the pressurized gas.
If such an accumulator is to be employed at a location where the operating temperature differs from the temperature at the location where the accumulator is filled with gas, care must be taken to ensure that the gas pressure in the accumulator at the filling location is of such a value that the correct gas pressure is obtained when the accumulator is at the operational location and has attained the operating temperature.
If the actuator""s and the accumulator""s operational location is a long way below the supply location where fluid is delivered to the actuator and return fluid received from the actuator, which, for example, is the case if the operational location is at a great depth in the ocean and the supply location is on a platform at the surface, and the actuator is not operated, the pressure in the return and supply lines at the operational location corresponds to the static pressure for a hydraulic fluid column with a height corresponding to the water depth. If the pressure of the fluid which is delivered to the accumulator is increased for operation of the actuator, and the actuator""s movable part, such as a piston, is moved, the pressure in the return line also rises as a function of the piston""s movement parameters and the fluid resistance in the return line.
The accumulator must therefore be adapted to both the relevant ocean depth and the actuator""s operating pressure as well as the temperature at this depth, and this represents a disadvantage.
An object of the invention is to provide a device in a subsea system for controlling a hydraulic actuator, and a subsea system with such a device, where the response time for the actuator is satisfactorily short, where the cross sectional dimensions for the return lines can be made acceptably small, and where the above-mentioned disadvantages when using a return accumulator are eliminated.